1. Field of the Invention
This invention relates to a compact zoom lens having a wide angle of field suitable for use in a digital still camera, a video camera, a camera for film, etc., and particularly to a zoom lens excellent in portability in which the widening of the photographing angle of field is achieved and of which the shortening of the full length is achieved.
2. Description of Related Art
Recently, with the higher function of cameras (optical apparatuses) such as a video camera using a solid state image pickup device, a digital camera and an electronic still camera, the compatibility of high optical performance and compactness has been required of optical systems using the same.
In a camera of this kind, it is necessary to dispose various optical members such as a low-pass filter and a color correcting filter between a lens rearmost portion and an image pickup device and therefore, a lens system having a relatively great back focal length is required of an optical system used therein. Further, in the case of a camera using a color image pickup device, in order to avoid color shading, as an optical system used therein, one which is good in the telecentric characteristic on the image side from which the exit pupil position is distant is desired.
As a means for achieving these requirements, there have heretofore been proposed various two-unit zoom lenses of the so-called negative lead type which comprise two lens units, i.e., a first lens unit of negative refractive power and a second lens unit of positive refractive power, and in which focal length change is effected by varying the interval between the two lens units. In these zoom optical systems of the negative lead type, the second lens unit of positive refractive power is moved to thereby effect focal length change, and the first lens unit of negative refractive power is moved to thereby effect the correction of an image point position resulting from the focal length change. In the lens construction comprising these two lens units, the zoom magnification is about double.
Also, among zoom lenses used in photographing apparatuses (optical apparatuses) using CCD such as video cameras, there is a lens type of three to five lens units in which the lens unit most adjacent to the object side begins with lens units of positive, negative and positive refractive power fixed during zooming.
For example, in Japanese Patent Application Laid-Open No. 63-81313 (corresponding U.S. Pat. No. 4,802,747), there is proposed a zoom lens having four lens units of positive, negative, positive and positive refractive power and having a variable power ratio of the order of three times. Also, as a zoom lens of a higher variable power ratio, for example in Japanese Patent Application Laid-Open No. 3-296706, there is proposed a zoom lens having four lens units of positive, negative, positive and positive refractive power and having a variable power ratio of the order of ten times.
On the other hand, as a zoom lens for an electronic still camera for photographing still pictures by the use of a CCD, there is desired an optical system very short in the full length of the lens and having, in terms of the characteristic of still pictures, a wide angle of field and having higher optical performance than a zoom lens used in a video camera for moving pictures.
As a lens having a relatively low variable power ratio of the order of 2.5 to three times as described above, but covering a wide angle area and bright and capable of obtaining high performance, there is proposed in Japanese Patent Publication No. 6-66008 (corresponding U.S. Pat. No. 4,662,723), etc. a zoom lens having two lens units of negative and positive refractive power and changing in focal length by a change in the air space between the lens units.
Further, in order to make an entire lens up to a shape having double or higher variable power ratio and yet compact, there are proposed, for example, in Japanese Patent Publication No. 7-3507 (corresponding U.S. Pat. No. 4,810,072), Japanese Patent Publication No. 6-40170 (corresponding U.S. Pat. No. 4,647,160), etc. so-called three-unit zoom lenses in which a third lens unit of positive refractive power is disposed on the image side of a two-unit zoom lens to thereby effect the correction of aberrations occurring with the tendency toward a higher variable power ratio.
In Japanese Patent Application Laid-Open No. 7-52256, there is proposed a zoom lens which comprises three lens units of negative, positive and positive refractive power and in which the spacing between the second lens unit and the third lens unit widens during the zooming from the wide angle end to the telephoto end.
In U.S. Pat. No. 5,434,710, there is disclosed a zoom lens which comprises three lens units of negative, positive and positive refractive power and in which the interval between the second lens unit and the third lens unit decreases during the zooming from the wide angle end to the telephoto end.
In Japanese Patent Application Laid-Open No. 60-31110 (corresponding U.S. Pat. No. 4,687,302), there is proposed a zoom lens which comprises four lens units of negative, positive, positive and positive refractive power and in which the interval between the second lens unit and the third lens unit decreases during the zooming from the wide angle end to the telephoto end and the fourth lens unit is fixed during the zooming.
In Japanese Patent Application Laid-Open No. 10-104520, there is disclosed a zoom lens which comprises three lens units of negative, positive and positive refractive power or four lens units of negative, positive, positive and positive refractive power.
In Japanese Patent Application Laid-Open No. 11-23967 (corresponding U.S. Pat. No. 6,124,984), there is disclosed a zoom lens which comprises three lens units of negative, positive and positive refractive power and in which the second lens unit is divided into two lens units 2a and 2b and which has a variable power ratio of about three times beyond the air space between the two. In Japanese Patent Application Laid-Open No. 11-84243 (corresponding U.S. Pat. Nos. 6,191,896 and 6,233,099), there is disclosed a four-unit zoom lens which comprises four lens units of negative, positive, positive and positive refractive power and in which a stop is disposed rearwardly of the second lens unit.
In recent years, solid state image pickup devices have been advanced toward more pixels, and a pixel size at a particular image size tends to become smaller. Along with this, as a photo-taking lens, there is required one having higher optical performance as compared with a conventional one of the same image size.
Accordingly, in the design limitation that the securing of a lens back (back focal length) necessary to insert a filter or the like rearwardly of a lens system and the telecentric characteristic of the emergence side (the exit pupil position lying at a great distance (infinity)) are compatible, it has become more difficult to realize a zoom lens of which the full length is shortened and which is compact and has a high variable power ratio.